NORTH ANDOVER — Television viewers who have endured even a single commercial break lately could be excused for thinking the bitter contest between US Representative John F. Tierney and Republican challenger Richard Tisei revolves around a single issue: how much, if anything, Tierney knew about the illegal gambling ring run by his wife’s brothers.

Neither said a word about that matter Wednesday night in their third debate, avoiding it just as they had in the first debate, and as they nearly did in the second, when only Tierney brought it up to accuse Tisei of an underhanded attack.

The raucous night saw the crowd frequently interrupting the candidates and even drowning out much of Tierney’s closing statement, first with boos from Tisei partisans and then cheers from Tierney supporters in the North Andover High School auditorium.

“I’m not even sure if we have a vote in this district, because he votes 99 percent of the time with the leadership,” Tisei said. “I’m going to look at every single issue individually, and I’m going to try to do the best thing I can for the people of this district. I’m a problem solver and an independent thinker.”

“Mr. Tisei is not registered as an independent, he’s not running as an independent, he’s running as a Republican. He’s running as a Tea Party Republican who has associated himself with . . . ” Tierney said, as boos raining down from the audience blotted out his words. He spoke through the catcalls until they receded, giving way to quiet before he was drowned out by his own supporters.

“If anybody thinks that [Tisei’s] going to change that dynamic, then they haven’t watched his competitiveness at the State House in all the 26 years he’s been there. We need to make sure that we keep moving in the right direction, and we need to have somebody who’s prochoice, for equality, for access to health care for women who will fight to protect Medicare, fight to . . . ” the congressman said, in the only audible part of his final statement.

The presence of a third candidate, Libertarian Dan Fishman, and a format in which the candidates took questions from a moderator — Sal Lupoli, owner of the Sal’s Pizza chain and Merrimack Valley Chamber of Commerce president — without direct interaction did little to mute the combativeness.

The debate was the third of six meetings between the two candidates.

Tierney opened not with a statement about himself but with a jab at Tisei for failing to agree to an earlier debate in this corner of the district proposed by the League of Women Voters, although both candidates have contributed to some scheduling hiccups.

And though Tisei is a self-described “live-and-let-live Republican” — fiscally conservative, socially libertarian — who notes that he has not taken conservative activist Grover Norquist’s anti-tax pledge, Tierney pointed out that Tisei did take the pledge two years ago, when he was Charlie Baker’s running mate for lieutenant governor.

In one skirmish, Tierney described his attempts to close tax exemptions for the wealthy and certain businesses as part of a “balanced approach” to the budget and economic recovery that would also include spending more on infrastructure.

Tisei scoffed.

“I would just ask the Congress if the bill ended up passing,” he said. “Did you reach across the aisle?”

“I couldn’t find a Republican that wanted to cut the tax exemptions if my life depended on it,” Tierney replied.

“Sorry, it’s my time,” Tisei interjected. “You’ve been in Congress for 16 years. You’re the only member of the Massachusetts congressional delegation who’s never sponsored a bill that’s been signed into law.”

Tisei cited his own bipartisanship in passing bills as a state senator from Wakefield, and in voting with Mitt Romney’s position as governor on just 54 percent of legislation.

Tierney said he had helped draft numerous bills that had been incorporated into successful legislation but that lacked his name in the final version, supporting lower interest rates for student loans, investment in green jobs, and a requirement for insurance companies to pay a refund when found to have spent too much on overhead.

“Richard, if we did laws the way you think, where everybody has to have it named after themselves to count, it would be sort of strange,” he said, citing “a number of substantial bills that I’ve worked on,” despite Republican efforts to foster a “do-nothing Congress.”

Although Tierney is an eight-term incumbent, the Sixth Congressional District race has become unusually competitive — and personal.

Tierney’s wife, Patrice, last year served a one-month prison sentence and five months of house arrest after admitting “willful blindness” to an illegal gambling enterprise run by her brothers. She agreed to plead guilty to tax fraud for handling bills and taxes in the United States for one of the brothers.

The issue has been the subject of ads on both sides.

Tierney and Tisei will meet again Thursday, appearing live on NECN’s “Broadside with Jim Braude” at 6 p.m.

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